There are many known power assist steering systems for automotive vehicles. Some provide steering assist by using hydraulic power and others by using electric power.
Electric assist steering systems include an electric motor drivably connected to the steerable vehicle wheels. When energized, the electric motor assists the steering movement of the steerable wheels. The electric assist motor is controlled in response to steering torque applied to the steering wheel.
Known electric assist steering systems typically include a D.C. permanent magnet electric assist motor electrically energized through an H-bridge drive circuit. One such drive arrangement is disclosed in U.S. Pat. No. 4,660,671 to Behr et al., an assigned to TRW Inc. In this arrangement, the H-bridge includes four field-effect-transistors ("FET's") connected in the "H" pattern. The electric assist motor is energized by turning one of the FET's continuously ON and pulse-width-modulating ("PWM") the diagonally opposed FET of the H-bridge. Current is controlled by varying the duty cycle of the PWM signal. The direction of current through the motor controls the direction in which the motor will rotate and, in turn, controls the steering direction.
It is desirable to use a variable reluctance motor in an electric assist steering system because of its small size, low friction, and its high torque-to-inertia ratio. The direction of rotation of a variable reluctance motor is controlled by controlling the sequence in which the stator windings are energized. Torque is controlled by controlling the amount of current through the stator windings. Accurate current control is necessary to produce low ripple torque, thereby reducing vibration felt at the steering wheel. One control arrangement for a variable reluctance electric assist motor is disclosed in U.S. Pat. No. 5,257,828 to Miller et al., and assigned to TRW Inc., and which is hereby fully incorporated herein by reference. The '828 patent controls current in the assist motor by pulse-width-modulating a series connected solid state switch.
As with systems using DC electric motors, fly-back current in systems using a variable reluctance motor is typically controlled through fly-back diodes. The fly-back diodes provide a current path for the fly-back current that results from the collapsing magnetic field of a recently de-energized stator coil.
Also of concern in an electric assist steering system is the operating temperature of the switching devices used to control the current through the electric assist motor. The above-mentioned Behr et al. '671 patent discloses a temperature sensing arrangement that folds back the PWM control signal if the temperature of the switching FET's exceeds a predetermined value. Also, a current sense arrangement senses current through the motor and folds back current if the sensed current exceeds a predetermined current threshold value.
A further concern of an electric assist steering system is the operational integrity of the system. The '671 patent also teaches several self-diagnostic features of the system. These diagnostic features include a torque signal absolute limit test, a summed torque signal test, an excessive PWM test, and a direction test.